This invention relates to a spectrophotometer using near infrared energy to irradiate samples.
Spectrophotometry determines the concentration of components in a sample by measuring transmission of radiation through or reflection of radiation from a test samples at narrow wavelength bands.
Radiation generated by a light source enters the spectrophotometer through an entrance slit. The light passing through the slit illuminates an oscillating diffraction grating which disperses the light into a spectrum. The spectrum is imaged across an exit slit and a narrow wavelength component of the spectrum passes through the exit slit and illuminates a sample after passing through exit slit optics. As the grating oscillates the wavelength of the spectral component irradiating the sample is swept through the spectrum. Accurate sample analysis hinges on the consistency of the angular position of the spectrum dispersed from the grating and errors in spectral measurements can arise due to inconsistencies in the light source. For example, the light intensity across the entrance slit can vary due to hot spots in the lamp. The resulting uneven illumination of the slit causes an apparent shift in the position of the slit and in the spectrum dispersed from the grating. Consequently, in near infrared spectrophotometric analysis, wave shift error of up to two nanometers can sometimes occur when the lamp is changed, thus requiring recalibration of the instrument or otherwise reducing the accuracy of the analysis provided by the instrument.
The prior art has employed fiber optics to uniformly illuminate the entrance slit, with some success. However, the fibers attenuate the radiation from the light source, reducing the intensity of radiation illuminating the sample, thus reducing the signal to noise ratio in the output signal from the spectrophotometer.